Species of Paulownia are fast-growing trees that are planted in many countries in green spaces and agroforestry systems. We studied the influence of irrigation on the diameter growth of 2-year-old Paulownia tomentosa saplings planted in 3 m 9 3 m grids at Nazlou Campus of Urmia University, Iran. Completely randomized experimental design was used with four irrigation amount, viz.10, 15, 20 and 25 L, and three irrigation intervals, viz. 3-and 6-days, in 8 replications. During the growth season, we measured collar diameters of all saplings at the end of each month. Diameter growth declined with reduced irrigation amount and increased irrigation interval. The lowest average diameter growth(9 mm) was recorded for 10 L water at 6-day intervals and maximum average diameter growth(28.7 mm) was recorded for 25 L volume at 3-day intervals. The latter treatment resulted in significantly greater diameter growth than any other treatment(except20 L at 3-day intervals). More than 80 % of diameter growth was achieved in the first 3 months. Therefore, it is essential to irrigate with 25 L of water at 3-day intervals during the first three growth months to achieve maximum diameter growth for 2-year-old P. tomentosa saplings. We studied the influence of irrigation on the diameter growth of 2-year-old Paulownia tomentosa saplings planted in 3 m 9 3 grids at Nazlou Campus of Urmia University, Iran. Completely randomized experimental design was used with four irrigation amounts, viz. 10, 15, 20 and 25 L, and three irrigation intervals, viz. 3- and 6-days, in 8 replications. season, we measured collar diameters of all saplings at the end of each month. Diameter growth declined with reduced irrigation amount and increased irrigation interval. The lowest average diameter growth (9 mm) was recorded for 10 L water at 6-day intervals and maximum Average diameter growth (28.7 mm) was recorded for 25 L volume at 3-day intervals. The latter treatment resulted in significantly greater diameter growth than any other treatment (except20 L at 3-day intervals). More than 80% of diameter gro groves wth was achieved in the first 3 months. it is essential to irrigate with 25 L of water at 3-day intervals during the first three growth months to achieve maximum diameter growth for 2-year-old P. tomentosa saplings.